Several applications have been developed for derivatives of dicarboxylic acid in the fields of coatings, detergents, and corrosion inhibitors. As used herein the term "dicarboxylic acid" is intended to mean a dicarboxylic acid having 21 carbon atoms, but in some instances includes minor amounts of dicarboxylic acid of other molecular weights. The versatility these materials exhibit in meeting the requirements of a variety of product applications is evidenced by their widespread use in commerce.
It is known in the art to react conjugated linoleic acid with certain dienophiles or activated mono-olefins to produce various polyfunctional Diels-Alder adducts. It is also known that the reactivity of the conjugated linoleic acid is determined by its geometrical isomerism about the double-bond system; and that the preferred reactive isomer has a trans-trans configuration. As demonstrated by the article, "Polymerization, Copolymerization, and Isomerization", J. C. Cowan, The Journal of the American Oil Chemicals Society. Vol. Nov. 31, 1954, pp. 529-535, it has long been taught that the use of catalysts (such as iodine, sodium or potassium bisulfates, sulfur, selenium, noble metals, and the like) to isomerize the cis-trans isomers into the trans-trans state is necessary to induce these cis-trans isomers of conjugated linoleic acid to react in a Diels-Alder reaction.
The method preferred by industry for the production of dicarboxyic acid is taught in commonly assigned U.S. Pat. No. 3,753,968, which is hereby incorporated by reference. There, a fatty acid mixture containing both conjugated and non-conjugated linoleic acid is simultaneously reacted with acrylic acid in the presence of an iodine catalyst to produce a fatty acid mixture containing dicarboxylic acid. This mixture is subsequently distilled to recover a linoleic free fatty acid fraction and a dicarboxylic acid fraction.
At the time this process was patented, it was believed that the amount of dicarboxylic acid formed was approximately the same as the starting content of linoleic acid in the fatty acid mixture. In other words, that the dicarboxylic acid material left after distillation was about 92% pure dicarboxylic acid. However, subsequent improvements in analytical instrumentation and techniques came to show that about 10% of what had been believed to be dicarboxylic acid was, in actuality, a C-21 lactone. This lactone was formed by the cyclization of the secondary carboxylic acid with the double bond of the cyclohexene ring. The lactonization reaction can result from the interaction of iodine with the double bond at the temperatures employed in the dicarboxylic acid synthesis.
It is difficult to remove the C-21 lactone from the dicarboxylic acid due to their structural similarity. Repeated wiped-film distillations will remove the lactone, but the procedure is costly and the final yield of purified dicarboxylic acid is extremely low.
It is also possible to purify the crude dicarboxylic acid by distillation of its methyl or dimethyl ester, as taught in U.S. Pat. No. 3,753,968. However, this procedure has proven too difficult and expensive to be feasible at a commercial scale.
Thus, no commercially feasible process has previously emerged which would produce a dicarboxylic acid of higher purity than that obtained via the method taught in U.S. Pat. No. 3,753,968 --a purity of only 85%. As a consequence, the potential applications for dicarboxylic acid in the fields of lubricants, coatings, detergents, plasticizers, and corrosion inhibitors have always been limited by the presence of other substances in the reaction mixture.
Although the most extensive uses of dibasic acids are to be found in producing polymers, dicarboxylic acid (as currently produced) has little or no utility in this area. It is recognized that one needs a high percentage of chain-forming difunctional molecules in order to be able to make a high molecular weight polymer. As 15% of the current dicarboxylic acid mixture is monofunctional or trifunctional material, it is far too impure to be used in polymer production.
Therefore, it is the object of this invention to provide an economical process for producing a dicarboxylic acid of high purity. Other objects, features, and advantages will be evident from the following disclosure.